WASHINGTON — The Pentagon remains on guard for an assault by Syrian government and Russian forces against a rebel enclave in northwest Syria, watching closely a Russian ship flotilla deployed in the Eastern Mediterranean.

The Russian fleet of roughly 15 vessels is now within striking distance of Syria and the rebel-held enclave of Idlib, Pentagon officials said Thursday. Russian and Syrian forces have been conducting airstrikes for about a week at targets in that region.

On Wednesday, the U.N. Office for the Coordination of Humanitarian Affairs said some of those airstrikes have damaged or destroyed hospitals and other civilian locations.

“Similar to previous periods of increased hostilities, numerous attacks on civilian infrastructure, including hospitals and schools, in north-west Syria have been reported so far in September.,” the U.N. report said. “In comparison to the whole of August, available data indicates a threefold increase in aerial bombardment and accompanied by a significant escalation in shelling.”

The region is home to about 3 million people, of which an estimated 2.1 million need humanitarian assistance, the U.N. agency said. “This increase in hostilities has caused civilian deaths and injuries, as well as damage to vital civilian infrastructure, such as hospitals, schools and other service-related centers in north-west Syria,” the report said.

A hearing on U.S. strategy in Syria by Pentagon officials before the House Armed Services Committee was cancelled Friday, so lawmakers could return to their districts in advance of Hurricane Florence.

U.S. and other nations have warned Syria not to use chemical weapons in the expected assault against Idlib, as it did in at least two earlier military assaults.

“The first time around he lost 17 percent of his pointy nosed Air Force airplanes,” Defense Secretary James Mattis told Pentagon reporters this week, referring to Syrian President Bashar al-Assad. That was the U.S. strike on a Syrian airfield in April 2017 following a chemical weapons attack by the Assad government on residents in the northern Idlib province — the same area under attack now.

“He’s been warned,” Mattis said Tuesday. “And so we’ll see if he’s wised up.”

The Russian ships are the newest element in suspected pre-assault staging, Pentagon officials said Thursday.

The Russian navy force “has been steadily building a large contingent of naval assets close to Syria with more than a dozen ships, many of those Kalibr-missile-equipped,” Pentagon spokesperson Eric Pahon told CNN’s Ryan Browne. Kalibr long-range cruise missiles continually change altitude and direction after launching and speed up as they hone in on their targets. Consequently, the missiles are exceedingly difficult for anti-craft and anti-missile systems to detect and intercept.